Up to three-quarters of postmenopausal women complain of disturbed, fragmented sleep. Yet, remarkably few studies have investigated putative relationships between age-associated changes in sex hormone profiles and sleep disturbance. Sex hormone levels exhibit age- related changes, and continue to play a significant role across the lifespan. With aging, and specifically at menopause, levels of estrogen (E2) and testosterone (T) decrease significantly, while luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels increase significantly; these changes are exacerbated with increasing age. Evidence, obtained primarily from perimenopausal women, suggests that these hormonal changes are associated with sleep disturbance. Likewise, a limited number of studies focusing on the effects of hormone replacement therapy on postmenopausal women indicate that E2 plays a protective role against sleep disturbance, while higher levels of LH may induce thermoregulatory or other changes that are associated with poor sleep quality. A test of the hypothesis that the balance of nighttime levels of E2 and T to LH and FSH is predictive of sleep disturbance is the primary focus of the current proposal. Specifically, this study will investigate the relationship between sex hormone levels and sleep disturbance in three groups of women. Young (20-30 years) premenopausal women, older (60+ years) postmenopausal women, and older (60+ years) postmenopausal women on estrogen replacement therapy (ERT) will spend an adaptation and two baseline nights in a sleep laboratory. Polysomnographical sleep and body core temperature will be assessed each night and frequent plasma sampling of E2, T, LH, and FSH will occur on the third night. It is predicted that a higher ratio of E2 and/or T to LH and/or FSH will be observed in the young group and the older group on ERT, and that these two groups will exhibit significantly higher sleep efficiency and less fragmented sleep. This study is viewed as the necessary first step to establishing a program of research investigating sex hormones and sleep across the lifespan, and processes involved in age-related sleep changes in both men and women.